6. At least two episodes of acute exacerbations in the former 12 months, or no exacerbations in the former 24 months. (Acute exacerbations are defined as worsening symptoms requiring treatment with systemic steroids (oral or parenteral) or antibiotics, a visit to the emergency room, and/or admission to a hospital. Events separated by at least 21 days are considered as separate events of exacerbation.)